French riot police have fired tear gas as demolition teams moved in to start dismantling makeshift homes at the Calais migrant camp known as The Jungle.

Tear gas was used reportedly in response to stone throwers at the slum - home to around 4,000 people, many of whom are intent on reaching Britain.

British volunteers condemned the response from the French authorities as it emerged that water cannon had also been sent into the site.

Tanya Freedman, of Help Refugees, said: "We're very disappointed because the French interior minister Bernard Cazeneuve insisted in a public statement that the clearance would be done in a humane fashion.

"On the first major day of dismantling, this is the way they are going about things."

Lines of police vans gathered on the perimeter of the southern section and people were prevented from going on to the site.

The action came after a judge in Lille ruled last Thursday that a partial clearance should go ahead, apart from social spaces including schools and places of worship.

French authorities want to relocate people from the squalid, rat-infested site to heated containers nearby or to centres around France.

Campaigners had called for a postponement to remove people from the slum, saying there was not enough new accommodation for people to move to.

A spokesman for Doctors of the World said its medics had been unable to enter as the entrance was blocked by police. Basic supplies and care materials were being prepared for those evicted.

Its UK director, Leigh Daynes, said: "Dismantling a large part of the main Calais camp is irresponsible and inhuman. It completely ignores needs on the ground and will just traumatise people already close to the edge.

"It won't stop refugees coming to Calais nor make the thousands there already disappear. People will just be forced elsewhere to live in even more squalid conditions."

Elaine Oritz, of the Hummingbird Project, which has been supporting Calais migrants and refugees, also said volunteers were not being allowed into the camp.

She said teams started arriving at The Jungle at around 8am, telling people they had an hour to leave the southern section of the camp.

"By dispersing people elsewhere, it means all the protective factors given to the people (at The Jungle), including medical help, will be removed," she said.

Human rights organisation Liberty said political leaders should not be "looking away" from the plight of refugees at The Jungle, including unaccompanied children.

Some have criticised the decision to send in demolition teams, including Liberal Democrat leader Tim Farron, who said razing the camp was not the solution.

But British hauliers welcomed last week's court's judgment.

The Freight Transport Association (FTA) said disruption caused by migrants cost the UK freight industry an estimated £750,000 a day last year.

FTA officials said a solution needed to be found to protect the £89 billion of UK trade which passes through cross-Channel ports annually.

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